Nowadays, it is commonly seen that backlight modules are used for electronic devices with flat panel displays, which includes devices as small as hand-held palm pilots and as large as big-screen TVs. A typical backlight consists of a light source, such as Cold Cathode Fluorescent (CCFL) or Light Emitting Diodes (LED), a light guide, a diffuser, and a brightness enhancement film. The design challenge of a backlight module is to generate uniform illumination across the LCD surface and luminance that is high enough to produce good contrast in a day environment (so that you can see the display of a laptop computer or a handheld electronic device, for example, with the room lights on), by the cooperation of the diffuser and the brightness enhancement film. Moreover, a conventional backlight usually consists of a plurality of optical films, which is the cause of low luminous efficacy, high manufacture cost and low assembly yield, and so on.
Please refer to FIG. 1, which is a perspective view of a luminance control film disclosed in U.S. Pat. No. 4,791,540, entitled “LIGHT FIXTURE PROVIDING NORMALIZED OUTPUT”. The luminance control film of FIG. 1 comprises a light source 10 and two lenticular sheets 12 and 14, being respectively disposed over the light source 10 for receiving light discharging therefrom; wherein the two lenticular sheets 12 and 14 is capable of collimating light incident thereto so as to enhance the direct luminance of the light fixture. However, the disclosed luminance control film is capable of collimating light effectively using the two lenticular sheets 12 and 14, it is disadvantageous by being able to diffuse incident light sufficiently enough to produce uniform illumination. Therefore, at least an additional diffuser is needed in the luminance control film of FIG. 1 for enabling the light fixture to produce uniform illumination. Thus, it is desired to have an optical film with both collimating and diffusion abilities without requiring the additional diffuser.
Please refer to FIG. 2, which is a typical assembly of the backlight module disclosed in U.S. Pat. No. 5,280,371, entitled “DIRECTIONAL DIFFUSER FOR A LIQUID CRYSTAL DISPLAY”. The assembly of FIG. 2 comprises a backlight array 20 with a plurality of lamps; a diffuser 22; a cylindrical lens array 24; and a liquid crystal panel 26; wherein, as the light discharging from the backlight array 20 impinges on the diffuser 22, it is scattered and thus blurring the image of the lamps; and thereafter, the scattered light is enhanced by the cylindrical lens array 24 so as to enable the light energy to concentrate on the liquid crystal panel 26. The cylindrical lens array has its pros and cons with respect to obtain the required concentration of luminance and variation of luminance with viewing angles, which is also true of the triangle lens array. Therefore, it is desired to have an optical film having cylindrical array and prism array formed thereon, which not only can simplified the structure complexity of the backlight module, but also can obtain a preferred optical effect by the combination of the two lens arrays.
Therefore, it is in great demand to have a direct backlight module having an optical film with micro-structures of diffusion and collimation abilities formed thereon, that it is capable of enhancing luminous efficiency, and further, it has a comparatively simple structure so as to reduce the manufacturing cost thereof.